1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, and laser beam printers wherein there is applied electrostatic recording system or electrophotographic system in which the electrostatic image formed on an image bearing member is developed by the use of a developer containing a toner and a carrier.
2. Description of the Related Art
A two-component developer prepared by mixing principally a non-magnetic toner with a magnetic carrier is widely used (hereinafter referred to as “two-component developing system”) as a developer used in an image forming apparatus due to the increase in quality and speed in full-color image forming apparatuses in recent years.
As a developing method wherein a two-component developer is used, there is a method wherein a developer containing a toner and a carrier is blended by means of a stirring/blending member, and the resulting blend is fed to the surface of a developer support. The developer support fixedly houses a magnetic roll in which a plurality of south poles and north poles are disposed alternately with each other, and the developer is in a condition wherein the developer forms a brush outline on the surface of the developer support by the magnetic force (hereinafter referred to as “magnetic brush”). Then, the developer magnetic brush supported on the surface of the developer support is allowed to be in contact with or to approach the surface of a photosensitive member, and a development bias voltage is applied between the developer support and the photosensitive member, whereby the toner is allowed to adhere onto an electrostatic image to complete the development.
In the case that a two-component developer is used in a reversal development system, an electrostatic force appears due to an electric potential difference between an image portion electric potential (V1 electric potential) and a development bias voltage (Vdc electric potential) applied on the developer support (hereinafter referred to as “development potential”). In the case that the electrostatic force is higher than that by which the carrier and the toner combine with each other, the toner leaves from the carrier. As a consequence, the toner adheres on the photosensitive member to effect the development.
At that time, such an electrostatic force which has a tendency to adhere onto the photosensitive member acts also by the electrostatic force due to the electric potential difference between the potential on the photosensitive member and the development bias voltage with respect to the carrier on the developer support. Consequently, the electrostatic force controls the electric potential on the surface of the photosensitive member and the developing bias voltage in such that the carrier stays on a development sleeve by means of the magnetic force of the developer support. Namely, in a normal state, the electrostatic force having the tendency wherein the carrier adheres onto the photosensitive member is the maximum in the surface potential area in a non-image portion (Vd electric potential) being an area charged; however, a higher magnetic force than the above-described electrostatic force is applied to the development sleeve, whereby it may be arranged in such that the carrier does not adhere to the photosensitive member.
On one hand, since a mixture ratio of a toner and a carrier in a development device (hereinafter referred to as “toner density”) varies depending on the consumption of the toner in the development device in which a two-component developer is used, it is necessary to maintain always the toner density at an adequate level. In the case that the toner density is inadequate, there is a case where an inadequate image such as image density variation, fog, or beads carry over appears. For this reason, it is important to control properly the toner density in view of forming a high-quality and highly stabilized image. A control method for toner supply includes, for example, a method for applying toner density detection unit (toner density detection system) such as a light detection system or an inductance detection system, and a patch detection system (image density detection system).
The problems appeared in the case that the above-mentioned two-component development system is applied are described hereinafter.
In the case that the charge amount of a toner in a development device increases with the use of an image forming apparatus, the charging amount of a carrier due to the countercharge of the toner is increased accordingly. As a result, there is a case where the force due to an electric field increases more than the magnetic force of a magnetic roll, so that the carrier is developed in the non-image area on an electrostatic image retaining member, resulting in beads carry over. In the case that the beads carry over appears as described above, when a developed toner layer on a photosensitive member is transferred to a transfer material such as a paper in a transfer portion, the carrier is transferred in the condition wherein the carrier is developed on the paper as it stands, and it results in decrease in image quality. Furthermore, duration of life decreases due to damaging the surface of a photosensitive member in the case that the photosensitive member is cleaned by means of a cleaning member, besides decrease in image quality and decrease in duration of life occur due to streaks produced by damaging a cleaning member, a transfer device, or a fixing device.
Under the circumstances, there are technologies for solving the above-described problems (For example, see Japanese Patent Application (JP-A) Nos. 2006-119380, H05-66678, and 2003-57939).
In JP-A No. 2006-119380, there is proposed an image forming apparatus which detects the presence or absence of beads carry over on a photosensitive member by means of an optical sensor, and allows the photosensitive member to isolate from a transfer device based on the detection result. Other than that, there are proposed a variety of image forming apparatuses which detect the beads carry over on a photosensitive member by means of an optical detecting unit.
In JP-A No. H05-66678, there is proposed an image forming apparatus wherein an electromagnet is provided on the downstream side of the transfer direction of a photosensitive member, and the electromagnet being in a roller-shaped and adapted to be rotated attracts the carrier adhered on the photosensitive member to remove the carrier.
In JP-A No. 2003-57939, there is proposed that the occurrence of beads carry over is predicted in the case that a toner density detection result in a development device is lower than a certain threshold value to stop an image forming operation in the image forming apparatus wherein a toner density detection system such as an optical detection system, or an inductance detection system is applied.
However, the optical detection unit as disclosed in JP-A No. 2006-119380 exhibits poor detection accuracy of beads carry over, and further cannot detect the whole area of a photosensitive member in the main scanning direction thereof. Consequently, there are a variety of problems such that a carrier cannot be detected other than at the positions where optical sensors are provided, so that the proposed optical detection unit has been not useful.
The unit as disclosed in JP-A No. H05-66678 is a unit for only removing a carrier from a photosensitive member, there is a limit in the recovery capacity in the case that beads carry over increases further depending on a development condition, and consequently, there is also such a fear that the carrier adhered cannot be recovered. Besides, the unit does not suppress fundamentally beads carry over in a development device.
In addition, since the technique of JP-A No. 2003-57939 does not detect actual beads carry over, there is such fear that a large amount of beads carry over has already existed at the time when the image forming operation is stopped, so that the technique has not been useful.